The overall objective of this proposal is to determine the temporal and spatial relationships existing between anatomically and functionally related neurons in the developing orbitofrontal cortex, caudate nucleus, and mediodorsal thalamic nucleus of the puppy. Both light microscopic silver degeneration and autoradiographic methods will be used to investigate the sequence of axonal development of neurons in these regions. Horseradish peroxidase methods will be employed to determine cell types giving rise to specific terminal projection fields. The Golgi method will be used to study the rates of dendritic and somatic maturation of these neurons and the relationships of this development to the arrival of afferent fibers. Standard transmission electron microscopy as well as combined electron microscopic-autoradiographic and Golgi-electron microscopic methods will be used to determine the precise temporal and spatial parameters of synaptogenesis in these regions.